In such objectives it is customary to assemble the varifocal front group from two axially shiftable components of negative refractivity bracketed by two positively refracting components whose axial position is substantially fixed (allowance being made for a limited axial adjustability, for focusing purposes, of especially the first component or at least a front lens forming part thereof). A typical varifocal or pancratic objective of this description has been disclosed in U.S. Pat. No. 3,433,559 and in our copending application Ser. No. 713,224 filed Aug. 10, 1976.
There are certain relationships in such a system between their optical qualities, such as relative aperture and varifocal ratio, and their structural parameters including front-lens diameter and total axial length. These relationships are well understood in the art; reference may be made, by way of example, to Austrian Pat. No. 317,576.
It is generally convenient to design the varifocal front group as an afocal system in order to allow interposition of ancillary elements, such as a shutter, a diaphragm and a reflex prism, in a field of parallel rays between the two groups. Since the fourth component of the front group is positively refracting, the combination of the three preceding components must be of dispersive character so that axially incident rays diverge from the axis between the third and fourth components. The extent of this divergence increases, of course, in proportion to the axial separation of the two components; an axial ray bundle fully illuminating the stationary fourth component passes, therefore, through a smaller portion of the third component when the latter is in a forward position than when it lies close to the fourth component.
Systems are known (e.g. from Austrian Pat. Nos. 317,576 and 316,891) in which the axial movement of the third component, designed to keep constant the image plane of the objective during changes of the overall focal length, follows a substantially symmetrical course as the second component is shifted rearwardly from a wide-angle position of minimum focal length f.sub.min to a telephoto position of maximum focal length f.sub.max. In such a case, the third component moves forwardly during a changeover from f.sub.min to an intermediate focal length f.sub.med and then recedes at about the same rate during a changeover from f.sub.med to f.sub.max, being substantially equidistant from the fourth component in the two limiting positions of the varifocal range.
Alternatively, e.g. as known from French Pat. No. 1,459,076 and German published Specification No. 2,164,937, the motion of the third component may be an unsymmetrical function of the displacement of the second component, so chosen that the third component approaches the fourth component more closely in the telephoto position (f.sub.max) than in the wide-angle position (f.sub.min).